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That's the idle position switch.
It can inform the controller that the pedal has fully returned. This is important because the physical structure around the TPS (throttle position sensor or "pot box") should set the physical travel limit, not the potentiometer itself. This makes it appear that the pedal is always slightly pressed. When the switch closes, the controller knows that the motor should be at idle/off.
A software-based controller can use the resistance/voltage/current value read as the baseline for "go pedal" actuation, while an old analog controller can use it for an easy "high pedal lockout" implementation.
Good controller software should allow hooking this switch up, and occasionally update the idle value to handle any change in the TPS response due to heat. Without a 'closed throttle' switch the controller has to be a little smarter: keeping a "deadband" where it doesn't activate the motor when it appears there is a slight pedal application, and updating its idea of the full-off position if this condition persists.
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